1. Field of the Invention
The present invention relates to a wire-cut, electric-discharge machining power source, and more particularly to a wire-cut, electric-discharge machining power source which is capable of suppressing electrolytic action on a workpiece and which permits high-speed machining.
2. Description of the Prior Art
A wire-cut, electric-discharge machine has various advantages such as requiring no special operator skills for working a metal mold or the like of a complicated configuration with a high degree of accuracy and, since the electrode for the electric-discharge machining is a wire, there is no need to manufacture a working electrode having required configuration for a metal mold as is needed in ordinary electric-discharge machines. On account of such advantages, the wire-cut, electric-discharge machine is has very wide application, but there is a drawback to because machine that the cutting speed is low. In order to increase the cutting speed, the machine has undergone various improvements. Also the present inventor has previously proposed, as the wire-cut, electric-discharge machining power source, a power source of the type in which a voltage is applied from a low-voltage, low-current power source to a machining gap defined between the wire and a workpiece to trigger a discharge and then a pulse current of large current value and small pulse width is applied from a high-voltage, high-current power source to the aforesaid gap, thereby to increase the cutting speed.
With the aforesaid power source previously proposed by the present inventor, both the low-voltage and the high-voltage power source employ the workpiece as the anode and the wire as the cathode, as is the case with the prior art. Accordingly, in the case of a wire-cut, electric-discharge machine of the type employing water as a working liquid and producing a discharge in the water, the workpiece is hot-worked by the discharge and, at the same time, subjected to electrochemical machining by the electrolytic action. The electrolytic action generates hydrogen bubbles in the machining gap and discharge of the bubbles may break the wire; furthermore, the flatness of the worked surface is reduced by non-uniform conductivity distribution of the working liquid over the entire area of the work surface and the workpiece readily gets rusty. In the case where the workpiece is made of a hard metal containing cobalt as a binder, the cobalt is corroded, presenting a problem of material defects. Accordingly, it is desirable to minimize the electrolytic action on the workpiece.
It is not preferred, however, to use the workpiece as the cathode and the wire as the anode contrary to the arrangement usually employed, with the view of removing the electrolytic action. The reason is that such reverse-polarity discharge generally makes the discharge unstable (markedly decreasing the cutting speed) and greatly wears the wire (often breaking it).